Monday, November 8, 1999 Published at 09:16 GMTSci/TechCarbon tubes could store hydrogen fuelHydrogen-powered cars are already in developmentChinese and American scientists have developed a method of storing high quantities of hydrogen inside tiny tubes of carbon just two nanometres (billionths of a metre) across.

The development is another step in the search for technical solutions to the problems that currently prevent hydrogen from being used as a practical, everyday fuel.

The gas produces no pollution and no greenhouse emissions when burned in pure oxygen and is considered by many to be the clean energy of the future and a replacement for fossil fuels when current reserves run out.

But whilst its energy content on a mass-for-mass basis is better than petrol, hydrogen has difficulty competing with the fossil fuel because it is a gas. A hydrogen gas fuel tank that contained a store of energy equivalent to a petrol tank would be more than 3,000 times bigger than its conventional cousin. Compressing or liquefying the gas is expensive.

Current thinking points to the absorption of hydrogen in another medium. Metal alloys can be persuaded to absorb up to 1,000 times their own volume of hydrogen but they are heavy and become brittle after repeated use.

Now a team from the Chinese Academy of Sciences in Shenyang, China, and the Massachusetts Institute of Technology, Cambridge, US, has demonstrated an effective way to absorb hydrogen into carbon nanotubes - minute cylinders made of carbon atoms.

Wash and dry

Their research, published in the journal Science, substantially increases the amount of hydrogen that can be absorbed using this method.

It involves soaking the nanotubes in hydrochloric acid then heating them to 500 degC for two hours. This wash-and-dry procedure removes most of the impurities that have inhibited previous attempts to get carbon nanofibres to absorb hydrogen.

The team found that its tubes would absorb one hydrogen atom for every two carbon atoms. What is more, almost 80% of the stored hydrogen could be released from the tubes at room temperature and pressure, with the rest released after the tube was heated.

But hydrogen still has a long way to go before it becomes a real alternative to fossil fuels. For a start, manufacturing anything on the nano scale is currently hugely expensive. To produce one kilo of carbon nanotubes costs about $1m.

A cheap method of making hydrogen also needs to be found. The obvious route involves splitting water, but this in itself requires substantial amounts of energy. One possible alternative might involve the use of a bacterial enzyme that uses iron to make hydrogen.